In electrical insulating materials for motors, it has recently demanded to have heat resistance at high temperature and hydrolysis resistance. For example, from an environmental problem in connection with abolition of specific chlorofluorocarbons, new alternatives for cooling medium have been proposed for motors used in refrigerators and air conditioners, however, the alternative cooling medium and lubricant oil used for refrigerators and air conditioners easily absorb water. Therefore, there is a demand for hydrolysis resistance in addition to heat resistance for electrical insulating materials used for refrigerators and air conditioners. In electrical insulating materials for motors used in hybrid cars, water is infiltrated into the materials under usage environment, so there is a demand for hydrolysis resistance in addition to heat resistance.
As a diaphragm for acoustic equipment or the like made of plastic, polyethylene terephthalate (PET) film is used, and a diaphragm for acoustic equipment employing polyethylene naphthalate (PEN) or polyether imide (PEI) which has more excellent heat resistance and rigidity than that of PET is also used.
However, the diaphragm for acoustic equipment employing PET has insufficient heat resistance when used for a small diameter speaker such as a speaker for a cellular phone because the diaphragm is likely to thermally deforms at temperatures not lower than 65° C. On the other hand, although the diaphragms for acoustic equipment using PEN has a better heat resistance than those using PET, their heat resistance is not still sufficient. The diaphragm for acoustic equipment employing PEI has problems in that rolling or sound distortion is likely to occur depending on the shape of the speaker diaphragm so that the audio characteristics are poor, and when the external output is high, the film may not withstand the output and may break.
Since a polyarylene sulfide film has characteristics such as excellent heat resistance, flame retardancy, rigidity, chemical resistance, electrical insulation and low moisture absorption, application of the film to electrical insulating materials and to speaker diaphragms is now being developed. For example, (1) the biaxially oriented films are known to be employed as electrical insulating materials (see Japanese Unexamined Patent Publication No. 1980-35456). (2) As polyarylene sulfide films, acoustic diaphragm films made of PPS films are proposed (see Japanese Unexamined Patent Publication No. 1994-305019).
However, the conventional film and sheet described above have the following problems. That is, the film in the item (1) above may be unsatisfactory in tensile elongation at break, ductility, and when used for example as a motor slot liner or as a wedge, causes film breakage or the like in some cases. The film in the item (2) above is poor in tensile elongation at break and there is a problem in that film breakage occurs in the molding process.
As described above, the polyphenylene sulfide film is poor in tensile elongation at break and ductility, thus making its applications limited at present, and its improvement is strongly desired. For a method of improving its ductility, a resin composition or a film having other thermoplastic resin mixed in polyphenylene sulfide resin is proposed. For example, employing polyphenylene sulfide (hereinafter also referred to as “PPS”) as a polyarylene sulfide, a composition comprising nylon 11 and nylon 12 dispersed as particles having an average dispersion diameter of 1 μm or less (see Japanese Unexamined Patent Publication No. 1991-81367), a composition comprising PPS, polyamide and epoxy resin (see Japanese Unexamined Patent Publication No. 1984-155462), a composition comprising PPS and polyamide (see Japanese Unexamined Patent Publication Nos. 1988-189458 and 2001-302918), a film comprising PPS and polyether imide (see Japanese Unexamined Patent Publication No. 1992-146935), a film comprising PPS and polysulfone (see Japanese Unexamined Patent Publication No. 1987-121761) or the like are disclosed, but they do not have sufficient film-forming stability, and substantially the improvements are not due to PPS alone. A biaxially oriented film in which metaphenylene sulfide units are copolymerized or blended in the polyphenylene sulfide (see Japanese Unexamined Patent Publication No. 1988-260426) is disclosed, but the heat shrinkage ratio of the area of the film is as high as 20% at 230° C. and the heat resistance of the film was not sufficient.
It could therefore be helpful to provide a biaxially oriented polyarylene sulfide film which has an excellent elongation at break and flatness.